1. Field of the Invention
The present invention relates to a variable-frequency type radio-frequency quadrupole accelerator for accelerating ions. More particularly, this invention is concerned with stabilization of a resonant frequency in a resonant circuit composed of a quadrupole and a secondary coil.
2. Background of the Invention
A radio-frequency quadrupole accelerator is a means for accelerating ions to produce high-energy ions. A variable-frequency type radio-frequency quadrupole accelerator (Japanese Patent Application Nos. 62-249400 and 3-245499, and U.S. Pat. No. 4,712,042) is known as a type of radio-frequency quadrupole accelerator enabling free variation of outgoing ion energy.
A conventional variable-frequency type radio-frequency quadrupole accelerator comprises a quadrupole composed of four elongated electrodes which are symmetrically arranged around a central axis and whose surface facing the axis undulates periodically, modulated, a secondary coil connected to the quadrupole and formed with a one-turn coil, and a primary coil magnetically coupled with the secondary coil. The secondary coil is connected to electrostatic capacitors constituting the quadrupole, thus forming a resonant circuit. The resonant circuit may be connected to a variable capacitor.
When power is supplied to the primary coil of the foregoing variable-frequency type accelerator over power supply lines, radio-frequency power is applied to the resonant circuit, which is composed of the electrostatic capacitors constituting the quadrupole and the secondary coil, via the secondary coil magnetically coupled with the primary coil. Consequently, ions are accelerated.
In general, the resonant frequency of a radio-frequency quadrupole accelerator is determined by the shapes of a quadrupole and a secondary coil. By generating a given voltage having a frequency equal to the resonant frequency among four electrodes of a quadrupole, ions are accelerated to an energy level dependent on three independent factors, that is, the structure of the modulation of the four elongated electrodes of the quadrupole, the resonant frequency, and the ion species. (I. M. Kapchinskii and V. A. Teplyakov, "Linear Ion Accelerator with Spatially Homogeneous Strong Focusing," UDC 621,385.64, "Nuclear Experimental Techniques," Plenum Publishing Corporation, New York, USA, 1970, p.322). Especially in a variable-frequency type radio-frequency quadrupole accelerator, the resonant frequency of the resonant circuit can be set to a desired value by, for example, changing the length or cross-sectional area of a secondary coil and thus varying the inductance of the secondary coil. Eventually, any given ions can be accelerated to various desired energies.
However, since a large amount of power is applied to the radio-frequency quadrupole accelerator, component members of the accelerator undergo thermal deformations due to an increase in temperature. This may cause the resonant frequency to vary. When the resonant frequency changes from a given value, one of the aforesaid factors determining ion energy varies. It therefore becomes impossible to accelerate given ions to the desired energy level.
As a means for solving the foregoing problem, a proposal has been made for a structure in which power is supplied to a quadrupole via a support column for supporting the quadrupole. In the structure, coolant is fed into the quadrupole and support column as well as the pedestal of the support column in order to cool them. Moreover, a cooling pipe is laid down along the support column, half embedded in the pedestal, and secured with a cooling pipe fixture (Japanese Patent Application Nos. 3-233844 and 4-61900).
However, in the foregoing prior art, the problem of preventing a thermal deformation of the secondary coil is not considered. The resonant frequency therefore varies due to a thermal deformation of the secondary coil. Given ions cannot therefore be accelerated to the desired energy level.
Impedance matching between a resonant circuit and power supply lines is attained by optimizing the shape and structure of a primary coil in a power line with the shape and structure of the resonant circuit. By attaining impedance matching, power supplied to the resonant circuit can be maximized.
However, impedance matching between a resonant circuit and power supply lines is impaired by the thermal deformations of a secondary coil forming the resonant circuit and of a primary coil. This poses the problem that power supply to the resonant circuit deteriorates.
An object of the present invention is to provide a variable-frequency type radio-frequency quadrupole accelerator capable of restraining thermal deformation of a secondary coil.
Another object of the present invention is to provide a variable-frequency type radio-frequency quadrupole accelerator capable of restraining an impedance change caused by thermal deformation of a power supply system.